According to this writer, the footage shows Thor and Loki fighting each other in front of Malekith--and Loki winning the fight!--before the, um, hand thing. Eek!

As I am new to this particular discussion regarding Thor's hand I will give my initial impression as to what I think is going on. I am limited in my knowledge of the details. Here goes. Loki and Thor are working in concert and it is Loki's magic that gives the illusion that Thor has lost his hand in their combat. This is to fool whoever it is they are fighting in front of.